1. Field of the Invention
The present invention relates to a solenoid valve, and, more particularly, to a normal open type solenoid valve applicable to an electronically controllable brake system for vehicles.
2. Description of the Related Art
FIG. 1 is a longitudinal sectional view showing the structure of a conventional normal open type solenoid valve 10, which is applied to an electronically controllable brake system for vehicles. As is shown in FIG. 1, the conventional normal open type solenoid valve 10 is operated to constantly open a flow channel when a vehicle is normally braked. When a driver steps on a brake pedal 11a, oil supplied from a master cylinder 11 is delivered to a wheel 12 of the vehicle via a filter 1, an orifice 2a formed at a valve seat 2, and a hollow part 3a and a communication hole 3b formed at a housing 3, by which the vehicle is braked.
When the vehicle is suddenly braked or braked on a slippery road, on the other hand, an electronic control system is operated such that electric current is supplied to a solenoid coil 4. When electric current is supplied to the solenoid coil 4, a magnetic force is generated between an armature 5 and the housing 3 by means of a magnetic field created at the solenoid coil 4 with the result that the armature 5 moves toward the housing 3. Consequently, a plunger 6 moves toward the valve seat 2 along with the armature 5, by which the orifice 2a is closed.
When electric current is not supplied to the solenoid coil 4, the plunger 6 and the armature 5 move back to their original positions by means of a resilient force of a restoring spring 7 such that the orifice 2a is opened. The above-described operations are repeatedly carried out to prevent slippage of the vehicle.
When the driver lifts his/her foot from the brake pedal 11a, the oil is returned from the wheel 12 to the master cylinder 11 via the communication hole 3b, the orifice 2a, and the filter 1, since the oil pressure at the wheel side is higher than that at the master cylinder side. As a result, the oil pressure at the wheel side is released, and thus the brake is released.
The return of the oil must be accomplished as soon as possible after the brake is released such that the wheel 12 is smoothly rotated. In the conventional solenoid valve 10, however, the diameter of the orifice 2a is small. As a result, the return of the oil via the orifice 2a is slow, and thus the oil pressure at the wheel side is not quickly released.
Especially when the orifice 2a is intermittently opened and closed as the electronic control system is operated to prevent slippage of the vehicle, the orifice 2a may be kept closed after the brake is released. For this reason, the conventional solenoid valve 10 further comprises an additional oil return channel and a check valve 8 in addition to the return of oil via the orifice 2a. The check valve 8 has a lip seal 8a, by which the oil is delivered from the wheel side to the master cylinder side. However, the lip seal 8a has a low durability, and the lip seal 8a may be easily deformed by abrupt fluctuation in pressure. Consequently, reliability of the conventional solenoid valve 10 with the above-stated construction is very low.